llvm/lib/Target/TargetLoweringObjectFile.cpp

   1 //===-- llvm/Target/TargetLoweringObjectFile.cpp - Object File Info -------===//
   2 //
   3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
   4 // See https://llvm.org/LICENSE.txt for license information.
   5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
   6 //
   7 //===----------------------------------------------------------------------===//
   8 //
   9 // This file implements classes used to handle lowerings specific to common
  10 // object file formats.
  11 //
  12 //===----------------------------------------------------------------------===//
  13
  14 #include "llvm/Target/TargetLoweringObjectFile.h"
  15 #include "llvm/BinaryFormat/Dwarf.h"
  16 #include "llvm/IR/Constants.h"
  17 #include "llvm/IR/DataLayout.h"
  18 #include "llvm/IR/DerivedTypes.h"
  19 #include "llvm/IR/Function.h"
  20 #include "llvm/IR/GlobalVariable.h"
  21 #include "llvm/IR/Mangler.h"
  22 #include "llvm/IR/Module.h"
  23 #include "llvm/MC/MCAsmInfo.h"
  24 #include "llvm/MC/MCContext.h"
  25 #include "llvm/MC/MCExpr.h"
  26 #include "llvm/MC/MCStreamer.h"
  27 #include "llvm/MC/SectionKind.h"
  28 #include "llvm/Support/ErrorHandling.h"
  29 #include "llvm/Target/TargetMachine.h"
  30 #include "llvm/Target/TargetOptions.h"
  31 using namespace llvm;
  32
  33 //===----------------------------------------------------------------------===//
  34 //                              Generic Code
  35 //===----------------------------------------------------------------------===//
  36
  37 /// Initialize - this method must be called before any actual lowering is
  38 /// done.  This specifies the current context for codegen, and gives the
  39 /// lowering implementations a chance to set up their default sections.
  40 void TargetLoweringObjectFile::Initialize(MCContext &ctx,
  41                                           const TargetMachine &TM) {
  42   // `Initialize` can be called more than once.
  43   delete Mang;
  44   Mang = new Mangler();
  45   initMCObjectFileInfo(ctx, TM.isPositionIndependent(),
  46                        TM.getCodeModel() == CodeModel::Large);
  47
  48   // Reset various EH DWARF encodings.
  49   PersonalityEncoding = LSDAEncoding = TTypeEncoding = dwarf::DW_EH_PE_absptr;
  50   CallSiteEncoding = dwarf::DW_EH_PE_uleb128;
  51
  52   this->TM = &TM;
  53 }
  54
  55 TargetLoweringObjectFile::~TargetLoweringObjectFile() {
  56   delete Mang;
  57 }
  58
  59 unsigned TargetLoweringObjectFile::getCallSiteEncoding() const {
  60   // If target does not have LEB128 directives, we would need the
  61   // call site encoding to be udata4 so that the alternative path
  62   // for not having LEB128 directives could work.
  63   if (!getContext().getAsmInfo()->hasLEB128Directives())
  64     return dwarf::DW_EH_PE_udata4;
  65   return CallSiteEncoding;
  66 }
  67
  68 static bool isNullOrUndef(const Constant *C) {
  69   // Check that the constant isn't all zeros or undefs.
  70   if (C->isNullValue() || isa<UndefValue>(C))
  71     return true;
  72   if (!isa<ConstantAggregate>(C))
  73     return false;
  74   for (const auto *Operand : C->operand_values()) {
  75     if (!isNullOrUndef(cast<Constant>(Operand)))
  76       return false;
  77   }
  78   return true;
  79 }
  80
  81 static bool isSuitableForBSS(const GlobalVariable *GV) {
  82   const Constant *C = GV->getInitializer();
  83
  84   // Must have zero initializer.
  85   if (!isNullOrUndef(C))
  86     return false;
  87
  88   // Leave constant zeros in readonly constant sections, so they can be shared.
  89   if (GV->isConstant())
  90     return false;
  91
  92   // If the global has an explicit section specified, don't put it in BSS.
  93   if (GV->hasSection())
  94     return false;
  95
  96   // Otherwise, put it in BSS!
  97   return true;
  98 }
  99
 100 /// IsNullTerminatedString - Return true if the specified constant (which is
 101 /// known to have a type that is an array of 1/2/4 byte elements) ends with a
 102 /// nul value and contains no other nuls in it.  Note that this is more general
 103 /// than ConstantDataSequential::isString because we allow 2 & 4 byte strings.
 104 static bool IsNullTerminatedString(const Constant *C) {
 105   // First check: is we have constant array terminated with zero
 106   if (const ConstantDataSequential *CDS = dyn_cast<ConstantDataSequential>(C)) {
 107     unsigned NumElts = CDS->getNumElements();
 108     assert(NumElts != 0 && "Can't have an empty CDS");
 109
 110     if (CDS->getElementAsInteger(NumElts-1) != 0)
 111       return false; // Not null terminated.
 112
 113     // Verify that the null doesn't occur anywhere else in the string.
 114     for (unsigned i = 0; i != NumElts-1; ++i)
 115       if (CDS->getElementAsInteger(i) == 0)
 116         return false;
 117     return true;
 118   }
 119
 120   // Another possibility: [1 x i8] zeroinitializer
 121   if (isa<ConstantAggregateZero>(C))
 122     return cast<ArrayType>(C->getType())->getNumElements() == 1;
 123
 124   return false;
 125 }
 126
 127 MCSymbol *TargetLoweringObjectFile::getSymbolWithGlobalValueBase(
 128     const GlobalValue *GV, StringRef Suffix, const TargetMachine &TM) const {
 129   assert(!Suffix.empty());
 130
 131   SmallString<60> NameStr;
 132   NameStr += GV->getParent()->getDataLayout().getPrivateGlobalPrefix();
 133   TM.getNameWithPrefix(NameStr, GV, *Mang);
 134   NameStr.append(Suffix.begin(), Suffix.end());
 135   return getContext().getOrCreateSymbol(NameStr);
 136 }
 137
 138 MCSymbol *TargetLoweringObjectFile::getCFIPersonalitySymbol(
 139     const GlobalValue *GV, const TargetMachine &TM,
 140     MachineModuleInfo *MMI) const {
 141   return TM.getSymbol(GV);
 142 }
 143
 144 void TargetLoweringObjectFile::emitPersonalityValue(MCStreamer &Streamer,
 145                                                     const DataLayout &,
 146                                                     const MCSymbol *Sym) const {
 147 }
 148
 149 void TargetLoweringObjectFile::emitCGProfileMetadata(MCStreamer &Streamer,
 150                                                      Module &M) const {
 151   MCContext &C = getContext();
 152   SmallVector<Module::ModuleFlagEntry, 8> ModuleFlags;
 153   M.getModuleFlagsMetadata(ModuleFlags);
 154
 155   MDNode *CFGProfile = nullptr;
 156
 157   for (const auto &MFE : ModuleFlags) {
 158     StringRef Key = MFE.Key->getString();
 159     if (Key == "CG Profile") {
 160       CFGProfile = cast<MDNode>(MFE.Val);
 161       break;
 162     }
 163   }
 164
 165   if (!CFGProfile)
 166     return;
 167
 168   auto GetSym = [this](const MDOperand &MDO) -> MCSymbol * {
 169     if (!MDO)
 170       return nullptr;
 171     auto *V = cast<ValueAsMetadata>(MDO);
 172     const Function *F = cast<Function>(V->getValue()->stripPointerCasts());
 173     if (F->hasDLLImportStorageClass())
 174       return nullptr;
 175     return TM->getSymbol(F);
 176   };
 177
 178   for (const auto &Edge : CFGProfile->operands()) {
 179     MDNode *E = cast<MDNode>(Edge);
 180     const MCSymbol *From = GetSym(E->getOperand(0));
 181     const MCSymbol *To = GetSym(E->getOperand(1));
 182     // Skip null functions. This can happen if functions are dead stripped after
 183     // the CGProfile pass has been run.
 184     if (!From || !To)
 185       continue;
 186     uint64_t Count = cast<ConstantAsMetadata>(E->getOperand(2))
 187                          ->getValue()
 188                          ->getUniqueInteger()
 189                          .getZExtValue();
 190     Streamer.emitCGProfileEntry(
 191         MCSymbolRefExpr::create(From, MCSymbolRefExpr::VK_None, C),
 192         MCSymbolRefExpr::create(To, MCSymbolRefExpr::VK_None, C), Count);
 193   }
 194 }
 195
 196 /// getKindForGlobal - This is a top-level target-independent classifier for
 197 /// a global object.  Given a global variable and information from the TM, this
 198 /// function classifies the global in a target independent manner. This function
 199 /// may be overridden by the target implementation.
 200 SectionKind TargetLoweringObjectFile::getKindForGlobal(const GlobalObject *GO,
 201                                                        const TargetMachine &TM){
 202   assert(!GO->isDeclarationForLinker() &&
 203          "Can only be used for global definitions");
 204
 205   // Functions are classified as text sections.
 206   if (isa<Function>(GO))
 207     return SectionKind::getText();
 208
 209   // Basic blocks are classified as text sections.
 210   if (isa<BasicBlock>(GO))
 211     return SectionKind::getText();
 212
 213   // Global variables require more detailed analysis.
 214   const auto *GVar = cast<GlobalVariable>(GO);
 215
 216   // Handle thread-local data first.
 217   if (GVar->isThreadLocal()) {
 218     if (isSuitableForBSS(GVar) && !TM.Options.NoZerosInBSS) {
 219       // Zero-initialized TLS variables with local linkage always get classified
 220       // as ThreadBSSLocal.
 221       if (GVar->hasLocalLinkage()) {
 222         return SectionKind::getThreadBSSLocal();
 223       }
 224       return SectionKind::getThreadBSS();
 225     }
 226     return SectionKind::getThreadData();
 227   }
 228
 229   // Variables with common linkage always get classified as common.
 230   if (GVar->hasCommonLinkage())
 231     return SectionKind::getCommon();
 232
 233   // Most non-mergeable zero data can be put in the BSS section unless otherwise
 234   // specified.
 235   if (isSuitableForBSS(GVar) && !TM.Options.NoZerosInBSS) {
 236     if (GVar->hasLocalLinkage())
 237       return SectionKind::getBSSLocal();
 238     else if (GVar->hasExternalLinkage())
 239       return SectionKind::getBSSExtern();
 240     return SectionKind::getBSS();
 241   }
 242
 243   // Global variables with '!exclude' should get the exclude section kind if
 244   // they have an explicit section and no other metadata.
 245   if (GVar->hasSection())
 246     if (MDNode *MD = GVar->getMetadata(LLVMContext::MD_exclude))
 247       if (!MD->getNumOperands())
 248         return SectionKind::getExclude();
 249
 250   // If the global is marked constant, we can put it into a mergable section,
 251   // a mergable string section, or general .data if it contains relocations.
 252   if (GVar->isConstant()) {
 253     // If the initializer for the global contains something that requires a
 254     // relocation, then we may have to drop this into a writable data section
 255     // even though it is marked const.
 256     const Constant *C = GVar->getInitializer();
 257     if (!C->needsRelocation()) {
 258       // If the global is required to have a unique address, it can't be put
 259       // into a mergable section: just drop it into the general read-only
 260       // section instead.
 261       if (!GVar->hasGlobalUnnamedAddr())
 262         return SectionKind::getReadOnly();
 263
 264       // If initializer is a null-terminated string, put it in a "cstring"
 265       // section of the right width.
 266       if (ArrayType *ATy = dyn_cast<ArrayType>(C->getType())) {
 267         if (IntegerType *ITy =
 268               dyn_cast<IntegerType>(ATy->getElementType())) {
 269           if ((ITy->getBitWidth() == 8 || ITy->getBitWidth() == 16 ||
 270                ITy->getBitWidth() == 32) &&
 271               IsNullTerminatedString(C)) {
 272             if (ITy->getBitWidth() == 8)
 273               return SectionKind::getMergeable1ByteCString();
 274             if (ITy->getBitWidth() == 16)
 275               return SectionKind::getMergeable2ByteCString();
 276
 277             assert(ITy->getBitWidth() == 32 && "Unknown width");
 278             return SectionKind::getMergeable4ByteCString();
 279           }
 280         }
 281       }
 282
 283       // Otherwise, just drop it into a mergable constant section.  If we have
 284       // a section for this size, use it, otherwise use the arbitrary sized
 285       // mergable section.
 286       switch (
 287           GVar->getParent()->getDataLayout().getTypeAllocSize(C->getType())) {
 288       case 4:  return SectionKind::getMergeableConst4();
 289       case 8:  return SectionKind::getMergeableConst8();
 290       case 16: return SectionKind::getMergeableConst16();
 291       case 32: return SectionKind::getMergeableConst32();
 292       default:
 293         return SectionKind::getReadOnly();
 294       }
 295
 296     } else {
 297       // In static, ROPI and RWPI relocation models, the linker will resolve
 298       // all addresses, so the relocation entries will actually be constants by
 299       // the time the app starts up.  However, we can't put this into a
 300       // mergable section, because the linker doesn't take relocations into
 301       // consideration when it tries to merge entries in the section.
 302       Reloc::Model ReloModel = TM.getRelocationModel();
 303       if (ReloModel == Reloc::Static || ReloModel == Reloc::ROPI ||
 304           ReloModel == Reloc::RWPI || ReloModel == Reloc::ROPI_RWPI ||
 305           !C->needsDynamicRelocation())
 306         return SectionKind::getReadOnly();
 307
 308       // Otherwise, the dynamic linker needs to fix it up, put it in the
 309       // writable data.rel section.
 310       return SectionKind::getReadOnlyWithRel();
 311     }
 312   }
 313
 314   // Okay, this isn't a constant.
 315   return SectionKind::getData();
 316 }
 317
 318 /// This method computes the appropriate section to emit the specified global
 319 /// variable or function definition.  This should not be passed external (or
 320 /// available externally) globals.
 321 MCSection *TargetLoweringObjectFile::SectionForGlobal(
 322     const GlobalObject *GO, SectionKind Kind, const TargetMachine &TM) const {
 323   // Select section name.
 324   if (GO->hasSection())
 325     return getExplicitSectionGlobal(GO, Kind, TM);
 326
 327   if (auto *GVar = dyn_cast<GlobalVariable>(GO)) {
 328     auto Attrs = GVar->getAttributes();
 329     if ((Attrs.hasAttribute("bss-section") && Kind.isBSS()) ||
 330         (Attrs.hasAttribute("data-section") && Kind.isData()) ||
 331         (Attrs.hasAttribute("relro-section") && Kind.isReadOnlyWithRel()) ||
 332         (Attrs.hasAttribute("rodata-section") && Kind.isReadOnly()))  {
 333        return getExplicitSectionGlobal(GO, Kind, TM);
 334     }
 335   }
 336
 337   if (auto *F = dyn_cast<Function>(GO)) {
 338     if (F->hasFnAttribute("implicit-section-name"))
 339       return getExplicitSectionGlobal(GO, Kind, TM);
 340   }
 341
 342   // Use default section depending on the 'type' of global
 343   return SelectSectionForGlobal(GO, Kind, TM);
 344 }
 345
 346 /// This method computes the appropriate section to emit the specified global
 347 /// variable or function definition. This should not be passed external (or
 348 /// available externally) globals.
 349 MCSection *
 350 TargetLoweringObjectFile::SectionForGlobal(const GlobalObject *GO,
 351                                            const TargetMachine &TM) const {
 352   return SectionForGlobal(GO, getKindForGlobal(GO, TM), TM);
 353 }
 354
 355 MCSection *TargetLoweringObjectFile::getSectionForJumpTable(
 356     const Function &F, const TargetMachine &TM) const {
 357   Align Alignment(1);
 358   return getSectionForConstant(F.getParent()->getDataLayout(),
 359                                SectionKind::getReadOnly(), /*C=*/nullptr,
 360                                Alignment);
 361 }
 362
 363 bool TargetLoweringObjectFile::shouldPutJumpTableInFunctionSection(
 364     bool UsesLabelDifference, const Function &F) const {
 365   // In PIC mode, we need to emit the jump table to the same section as the
 366   // function body itself, otherwise the label differences won't make sense.
 367   // FIXME: Need a better predicate for this: what about custom entries?
 368   if (UsesLabelDifference)
 369     return true;
 370
 371   // We should also do if the section name is NULL or function is declared
 372   // in discardable section
 373   // FIXME: this isn't the right predicate, should be based on the MCSection
 374   // for the function.
 375   return F.isWeakForLinker();
 376 }
 377
 378 /// Given a mergable constant with the specified size and relocation
 379 /// information, return a section that it should be placed in.
 380 MCSection *TargetLoweringObjectFile::getSectionForConstant(
 381     const DataLayout &DL, SectionKind Kind, const Constant *C,
 382     Align &Alignment) const {
 383   if (Kind.isReadOnly() && ReadOnlySection != nullptr)
 384     return ReadOnlySection;
 385
 386   return DataSection;
 387 }
 388
 389 MCSection *TargetLoweringObjectFile::getSectionForMachineBasicBlock(
 390     const Function &F, const MachineBasicBlock &MBB,
 391     const TargetMachine &TM) const {
 392   return nullptr;
 393 }
 394
 395 MCSection *TargetLoweringObjectFile::getUniqueSectionForFunction(
 396     const Function &F, const TargetMachine &TM) const {
 397   return nullptr;
 398 }
 399
 400 /// getTTypeGlobalReference - Return an MCExpr to use for a
 401 /// reference to the specified global variable from exception
 402 /// handling information.
 403 const MCExpr *TargetLoweringObjectFile::getTTypeGlobalReference(
 404     const GlobalValue *GV, unsigned Encoding, const TargetMachine &TM,
 405     MachineModuleInfo *MMI, MCStreamer &Streamer) const {
 406   const MCSymbolRefExpr *Ref =
 407       MCSymbolRefExpr::create(TM.getSymbol(GV), getContext());
 408
 409   return getTTypeReference(Ref, Encoding, Streamer);
 410 }
 411
 412 const MCExpr *TargetLoweringObjectFile::
 413 getTTypeReference(const MCSymbolRefExpr *Sym, unsigned Encoding,
 414                   MCStreamer &Streamer) const {
 415   switch (Encoding & 0x70) {
 416   default:
 417     report_fatal_error("We do not support this DWARF encoding yet!");
 418   case dwarf::DW_EH_PE_absptr:
 419     // Do nothing special
 420     return Sym;
 421   case dwarf::DW_EH_PE_pcrel: {
 422     // Emit a label to the streamer for the current position.  This gives us
 423     // .-foo addressing.
 424     MCSymbol *PCSym = getContext().createTempSymbol();
 425     Streamer.emitLabel(PCSym);
 426     const MCExpr *PC = MCSymbolRefExpr::create(PCSym, getContext());
 427     return MCBinaryExpr::createSub(Sym, PC, getContext());
 428   }
 429   }
 430 }
 431
 432 const MCExpr *TargetLoweringObjectFile::getDebugThreadLocalSymbol(const MCSymbol *Sym) const {
 433   // FIXME: It's not clear what, if any, default this should have - perhaps a
 434   // null return could mean 'no location' & we should just do that here.
 435   return MCSymbolRefExpr::create(Sym, getContext());
 436 }
 437
 438 void TargetLoweringObjectFile::getNameWithPrefix(
 439     SmallVectorImpl<char> &OutName, const GlobalValue *GV,
 440     const TargetMachine &TM) const {
 441   Mang->getNameWithPrefix(OutName, GV, /*CannotUsePrivateLabel=*/false);
 442 }